Discussion in 'Electronic Basics' started by Tim2cv6, May 25, 2004.

1. ### Tim2cv6Guest

I have recently bought a rev counter for an old car I am currently
restoring. The electronic problem I am having is the rev counter seems
to be reading about half of what it should be! Is there an easy fix to
this by means of changing resistors or am I stuck! I have taken the
back off it and can see a large resistor coloured brown with green,
black, pinky/orange? then silver banding. this is directly attached to
the red +ve input wire. I have pictures if needed to makes more sense.
Any ideas greatfully recieved!
TIM

2. ### John PopelishGuest

If that resistor is in series with a meter movement (not the
electronic circuit between the meter and the ignition system) then you
may be able to change it and recalibrate the meter. The resistor you
describe appears ot be a 15k ohm resistor (brown=1, green=5, orange=3,
so 15 followed by 3 zeros, silver=10% tolerance)

Before you remove this one, you might do an experiment. Parallel it
with a 47k resistor (yellow, purple, orange) and see if you get the
expected 11% or so increase in the meter reading. This experiment is
not likely to hurt anything if his is not the purpose of the resistor,
since it makes a total resistance change not much more than the
tolerance of the 15k resistor allows. If this works, then paralleling
the 15k with another 15k should almost double the reading.

3. ### JeBGuest

I'd begin by looking closer for something to select between 4, 6 and 8
cyl. engines.

4. ### Rob PaisleyGuest

How many cylinders does your car have?

Does the tachometer have an adjustment for the number of cylinders?

Rob.

5. ### Tim2cv6Guest

ok, thanks, another chain of thought would be to look at the signal
going to the rev counter from the electronic ignition. Is there any
way to convert the signal from 1 pulse to 2 pulses hence changing the
value of the signal to twice what it is at the moment. Thanks
TIM

6. ### ~^Johnny^~Guest

Ah.

Most simple Tachs are just freq counters, using a double diode-cap network.
It uses differentiating and integrating functions on the input pulses, to
yield an average voltage, which is read on the D'Arsonval meter. D'Arsonval
movements actually respond to current...
....now the problem is: these diode-cap counter type tachs assume twice the
frequency, due to the differentiating action. On an old style "kettering"
ignition system, this works fine, as the tack will double the frequency of
the incoming pulses off the points.. But with electronic ignition, the
waveform seen by the tach is radically different. The tach may no longer be
frequency doubling, so the average voltage seen by the
meter-movement-series-resistor combination might be halved. Either the
leading or trailing edge of the pulse train isn't abrupt enough, so the tach
sees only "half frequency".

It looks like the series resistor is 51 Kohms. So try another 51 Kohm
resistor in parallel with it. If little change, then it's 510 ohms, so use
a 510 ohm resistor across it instead.

Check the linearity when you're done. Simultaneously hook up a tach with
known accuracy (borrow one), and check it at three or four different speeds,
like curb idle, cruising, power band, and a little bit below redline.
--
-john
wide-open at throttle dot info

~~~~~~~~
"The first step in intelligent tinkering is to
save all the parts." - Aldo Leopold
~~~~~~~~

7. ### ~^Johnny^~Guest

AFTERTHOUGHT:

Also, the resistor could be 50K, 56K, 500 Ohm, or 560 Ohm.

Sometimes blue (6) can look black (0) or gray (8) if you are slightly color
blind, and don't know it, or if you are viewing it in poor lighting.
Fluorescent is generally the worst.

Here's what I want you to do:

Cut one of the leads of that resistor, right in the middle, and then measure
its resistance. You can later tack it back together easily enough with a bead
of solder. Then find another resistor / potentiometer to parallel it, to cut
its value in half.

--
-john
wide-open at throttle dot info

~~~~~~~~
Always listen to experts. They will explain what
can't be done and why. Then do it. - Robert Heinlein
~~~~~~~~

8. ### R. Steve WalzGuest

---------------
Send the signal to ground through a resistor, inline through a cap
off the top of that resistor to a second resistor to ground, then
full-wave rectify the resultant pseudo-AC voltage with an op-amp
fullwave rectifer that clips the bottom off with and use a Schmitt
trigger on the remaining pulses.

-Steve

9. ### ~^Johnny^~Guest

Why not just build a new &^##%@* tach from scratch? ;->
--
-john
wide-open at throttle dot info

~~~~~~~~
"The first step in intelligent tinkering is to
save all the parts." - Aldo Leopold
~~~~~~~~